In one known form of jacking system for use with thread bars, a pair of yoke frames is connected by a ram system for selectively moving the yoke frames toward and from each other in the jacking operation. Each of the yoke frames carries a cup plate and a rocker plate. When the thread bar and jacking device are moved relatively so that a nut or coupler on the thread bar is encountered, the cup plate and rocker plate must be removed by hand from the yoke frame encountering the nut or coupler. After the jacking device clears the nut or coupler, the cup plate and rocker plate are reinstalled.
While the prior art structure may be utilized to pull the thread bar with an attached load, the device is not adapted for use in hoisting the load by climing the thread bar. Further, the difficulty and expense of continuously removing and installing the cup plates and rocker plates makes the discussed prior art structure an unsatisfactory means for effecting such jacking of loads.